Diversity of responses to iron availability by phytoplankton of the California Current ecosystem

Coastal upwelling injects significant nutrients into California’s nearshore marine environment in a sporadic manner, which are subsequently advected offshore through a system of filaments and eddies. This imparts spatial and temporal heterogeneity to primary production by phytoplankton communities and leads to successional community dynamics as nutrient concentrations change quickly, favoring different functional groups. As nutrients are consumed, coastal diatom dominated communities frequently yield to picoeukaryotes, which are then displaced by Synechococcus, and finally Prochlorococcus offshore in the California Current. Limitation in the supply of bioavailable iron can exert significant bottom-up pressure across the region and elicits a variety of physiological responses from phytoplankton. These can include upregulation of iron acquisition proteins, substitution of iron-containing proteins with iron-free alternatives, modification of carbon and nitrogen metabolisms, and remodeling of the photosynthetic apparatus. We profiled these responses using metatranscriptomics from ambient seawater samples and in situ drifting iron and chelator addition experiments performed in 2014 across gradients in iron and macronutrient concentrations. Our results highlight the diverse strategies employed by the dominant phytoplankton groups to mitigate iron stress in different regimes within the California Current ecosystem.